1. Field of the Invention
The present invention relates to a method of conducting circumferential welding of an electric welded steel line pipe which is suitable to be laid by a reel barge.
2. Description of the Related Art
In one prior art this method, successive pipe modules are connected by circumferential welding conducted by manual welding, MIG (Metal Inert Gas) welding or MAG (Metal Active Gas) welding on a barge 1, and the continuous pipe composed of welded pipe segments is fed from the barge so as to be laid on the sea bottom. According to this method, however, it is necessary to conduct various operations on the barge, such as welding, inspection of the welded parts, coating and so forth. In general, such operations could not be conducted at high efficiency because only limited working space is available on the barge, and because the work on the barge is seriously influenced by weather and the condition of the sea.
In order to obviate these problems, a method called "reel barge method" is becoming popular. According to this method, circumferential welding of pipe modules, inspection and coating are conducted on shore so as to form a long continuous pipe. The thus formed long continuous pipe is wound on a reel mounted on the barge. After the barge has arrived at the place where the pipe is to be laid, the continuous pipe is unwound from the reel and laid on the sea bottom.
By using the reel barge method, it is possible to lay a submarine line pipe at a very high efficiency. This method, however, suffers from a problem in that tensile and compression stresses are generated in parts of the pipe when the pipe is wound on the reel or unwound from the reel so as to be laid on the sea bottom, tending to cause generation of cracks in the toe portion of the circumferential welded part of the base metal of the pipe.
Hitherto, seamless pipes have been used as the line pipes to be laid on the sea bottom, in view of the quality and strength. In recent years, however, attempts have been made to use electric welded steel pipes in place of seamless pipes, partly because the quality of electric welded pipes has been improved remarkably and partly because the use of this type of pipes is advantageous from an economical point of view.
Unfortunately, prevention of toe cracking in the circumferential weld portion is more difficult with the case of electric welded steel pipes than with the case of seamless pipes, for the following reason.
Namely, an electric welded steel pipe is usually formed from a steel sheet which undergoes a controlled rolling for attaining high strength and toughness. However, during the circumferential welding, the weld region is heated to a temperature above the AC.sub.3 transformation point to lose the effect of the controlled rolling, with the result that the heat affected zone is softened to a level below the hardness of the base metal, tending to exhibit a toe cracking.
Hitherto, various methods have been proposed to improve the toughness of the weld portions, such as those disclosed in Japanese Examined Patent Publication No. 60-31888 which teaches a post heat treatment after the welding while controlling the carbon equivalent of the base metal, and Japanese Unexamined Patent Publication No. 53-12751 which shows an application of supersonic vibration to the welded material during the welding. The method proposed in Japanese Examined Patent Publication No. 60-31888 requires the post heat treatment as an additional step. The method proposed in Japanese Unexamined Patent Publication No. 53-12751 requires that a separate device be provided for imparting vibration, as well as a difficult procedure for adjusting the vibration frequency since application of vibration at an inadequate vibration frequency tends to impair the toughness rather than improving the toughness.